Oil fired heating systems typically incorporate a primary burner control that controls the ignition and the supply of oil to the system burner. The primary burner control, in response to a signal from an aquastat or air thermostat, will activate the burner and supply oil to the burner. When the system reaches the desired temperature, the burner will shut off.
A common problem with oil fired heating systems is that over time sludge accumulates and settles to the bottom of the oil tank. Because the sludge settles below the pickup line, it does not typically create problems during regular operation of the heating system. However, during an oil delivery the sludge gets churned up when oil is added to the heating tank. If the burner control actives and supplies oil from the holding tank to the burner, the sludge will be drawn into the pickup tube to the burner. While the pickup line or burner usually includes a filter, this filter is only intended to prevent microparticles from reaching the burner. A significant amount of the sludge will clog the filter and prevent the fuel from reaching the burner, the burner flame will be extinguished and the system will shut down until the filter is cleaned or replaced.
A way to prevent clogging of the filter is to inhibit operation of the heating system during and directly after an oil delivery for a time sufficient to allow any sludge present in the holding tank to settle back down to the bottom of the tank.
U.S. Pat. No. 3,976,421 to Lee et al. discloses a sediment control apparatus as described above. The apparatus has an internal interrupt timer that incorporates a normally closed switch into the burner control circuit. The system also includes an initiation switch, connected to the timer, that an oil delivery person activates to start the interrupt timer. When the initiation switch is activated, the timer opens the normally closed switch to interrupt the control circuit.
A disadvantage of this device, however, is that the timer is integrated with and receives power from the same power supply as the burner control circuit. If the timer fails, for whatever reason, after being activated the normally closed switch will remain open. That will prevent operation of the heating system even after the elapsed interrupt period has passed, and the home will be without heat until the timer circuit can be repaired.